11_STA.md

Static Timing Analysis — Timing Is a Contract

Purpose of This Chapter

Static Timing Analysis (STA) is not a suggestion system.

It is a contract.

If STA says the design does not meet timing, the design is invalid, regardless of simulations or intentions.

What STA Actually Answers

STA answers one question only:

Will every signal arrive before it is required, under worst-case assumptions?

It does not simulate behavior. It does not care about functional correctness.

It only cares about time.

The Timing Contract

Every timing path is defined by:

• launch clock edge
• data path delay
• capture clock edge
• required arrival time

If:

arrival_time ≤ required_time

The path passes.

If not, the chip fails.

Key STA Terms (No Hand-Waving)

Setup Slack

• measures if data arrives too late
• negative slack = failure

Hold Slack

• measures if data arrives too early
• negative slack = failure

Both matter. Passing setup but failing hold is still failure.

Why Simulation Cannot Replace STA

Simulation:

• checks specific scenarios
• depends on testbench coverage
• ignores worst-case corners

STA:

• checks all paths
• assumes worst-case delay
• is corner-aware

Real silicon obeys STA, not simulation optimism.

Reading STA Reports Correctly

Never start with averages.

Always start with:

• worst negative slack (WNS)
• total negative slack (TNS)
• top failing paths

If WNS < 0: stop.

The Most Common STA Mistake

“It only fails by a little.”

There is no “a little” in silicon.

A −10 ps violation is as real as −1 ns.

Clock-Related Failures

Clock problems dominate STA failures:

• excessive clock skew
• poor CTS structure
• unbalanced clock trees

Fix clocks first. Always.

Data Path Failures

Common causes:

• long combinational chains
• excessive fanout
• poor placement
• routing detours

These are architectural problems, not tool problems.

Hold Violations Are Not Optional

Hold violations:

• cannot be masked
• cannot be fixed by frequency reduction
• must be fixed structurally

Ignoring hold violations guarantees silicon failure.

STA Before and After Routing

Pre-route STA:

• optimistic
• used for guidance

Post-route STA:

• reality
• final authority

Only post-route STA matters.

What STA Is Not

• not a tuning exercise
• not a report-generation task
• not optional
• not negotiable

STA is the law.

When to Roll Back

Immediately roll back if:

• WNS worsens after routing
• new violations appear unexpectedly
• CTS changes destabilize timing

Fix the cause, not the report.

Healthy STA Indicators

A healthy design shows:

• positive WNS
• small TNS
• stable results across runs
• predictable corner behavior

Anything else is a warning.

Why This Chapter Exists

Many designs “work” in simulation.

They still fail in silicon.

STA is how silicon speaks back.

Next Chapter

After timing is clean:

• 12_GLS.md — Logic Meets Reality